650 



ZOOLOGY 



SECT. 



groups, according as they are used for prehension, for creeping, 

 for running, or for swimming ; they end usually in two claws, 

 between which there may be discs or stalked suckers. 



In the Xiphosura or King-crabs (Fig. 558), the body consists of 

 two well-marked regions cephalothorax and abdomen. The former 

 is covered over by a wide, dorsally convex, sub-crescentic shield or 

 carapace, bearing two large compound eyes, and two smaller simple 

 eyes. The segments of the abdomen (seven in number) are united 



"together, being covered dor- 

 sally by a continuous abdo- 

 minal carapace. At the 

 posterior end is attached a 

 very long, narrow, caudal 

 spine or telson. The anterior 

 appendages (Fig. 559) re- 

 semble those of the Scorpion. 

 In front of the mouth is a 

 pair of short, three- jointed, 

 chelate appendages, the cheli- 

 cerce (1), at the sides of a 

 labrum (rostrum) or upper lip. 

 Behind these follow a series 

 of five pairs of legs, the bases 

 of all of which, with the 

 exception of the last, are , 

 covered with spines, and have 

 the action of jaws, while the 

 extremities are for the most 

 part chelate. The first pair 

 of appendages of the abdomen 

 are flat plates, which are 

 united together in the middle 

 line and together form the 

 broad operculum (operc.) over- 

 lapping all the posterior 

 appendages ; on its posterior 

 face are the two genital 

 apertures. The posterior 

 appendages, of which there 

 are five pairs, ^are thin flat plates to which the gills are attached ; 

 each of them is divided by a suture into a small inner ramus or 

 endopodite, and a larger external ramus or exopodite. Between 

 the sixth pair of appendages is a pair of processes, the chilaria. 

 In the Eurypterida (Fig. 560) there is a small cephalothorax 

 bearing a pair of large eyes and a pair of ocelli, and an elongated 

 segmented region containing twelve segments, followed by a 

 narrow pointed telson. There are usually five pairs of limbs 



FIG. 558. Limulus. Dorsal aspect. 

 (After Leuckart.) 



